The presence of contaminant particles on the surface of electronic substrates such as a semiconductor wafers, may lead to formation of defects during the microelectronics fabrication process. In order to maintain high manufacturing yield and thus low manufacturing costs, it is necessary that contaminated wafers be identified and cleaned during the manufacturing process. Further, the inspection of excursions and/or defects in the semiconductor wafers may also be used to detect any defects and/or quality issues within the wafer manufacturing processes.
ED-XRF is a reliable, sensitive and widely used technique for the detection and quantification of elemental concentrations within a sample material under investigation. Such a technique involves irradiating a surface of the sample material with a high energy beam, such as X-rays, from a source such as an X-ray tube and observing the resulting fluorescence emitted by the irradiated area through a radiation detector. The detector collects the emitted radiations and produces signals representative of the energies of received x-rays. Since each element has a different and identifiable X-ray fluorescence signature, an operator can determine the presence and concentration of the element(s) within the sample by measuring the wavelengths and intensity of the emitted radiation as well known in the art.
Generally, the ED-XRF technique provides a relative measurement and in order to arrive at a quantifiably correct concentration, a calibration method must be applied. Numerous efforts have been made to achieve an accurate calibration of the elemental concentration measurement within a sample semiconductor material.
In one such solution, known as reference standard for calibration, the sample material is pre-calibrated using reference samples of known quality having properties similar to the sample material being investigated. However, Since the ED-XRF measurements are sensitive to many factors, such as for example, chemical concentration of elements, the physical properties of samples, the influence of other elements present in the sample (also known as the inter-element effects) etc., the reference sample preparation is generally a time-consuming, expensive, and inconvenient process.
In another solution, known as Fundamental Parameters method for calibration, incorporates instrument parameters in the calibration equations using mathematical formulation of the elemental physical processes. While, such a method significantly reduces the effort for calibrating the measurements of xrf by theoretically yielding the composition and concentration of the elements present in the sample, the method is not considered accurate practically as it has limitations such as for example it assumes time invariance of the measurement system.
Further, with recent advancements, most of the semiconductor wafers are three dimensional structures having varying heights and thicknesses. The ED-XRF measuring system, in general, however, is not subtle enough to accurately measure the thickness etc. of the sample material to be investigated, as the emitted radiations from such sample materials will have different signal strength for different height areas. Accordingly, the signal representing the area having higher thickness may get saturated and the areas having very low height may suffer loss of sensitivity thereby not allowing the detector to measure an accurate elemental composition. Consequently, the elemental composition of material of different thickness cannot be appropriately differentiated by the XRF system alone.
Therefore, there is a need in art for a simple and accurate system that not only measures the three dimensional structure but also accurately calibrate the elemental composition of the material to be investigated.